The present invention relates to a sol solution and a method for film formation, and more particularly to a sol solution useful for the formation of a protective layer for a dielectric layer in an alternating current type plasma display and a method for film formation using the sol solution.
For optical components, electronic and electrical components, magnetic material components and the like, layers having various functions are formed on the substrate, and, if necessary, a protective film is provided on the surface of these layers for protection purposes. High film strength and large adhesion to the above functional layers are general properties required of such protective films.
In recent years, research and development of flat panel displays as an alternative to CRT have been energetically conducted. Among them, the so-called "plasma display" which utilizes luminous phenomenon accompanying discharge in a display is classified, according to the structure of the electrode (mainly ITO), roughly into a direct current type, wherein metallic electrodes are exposed to a discharge space, and an alternating current type wherein electrodes are covered with a dielectric layer. In the latter alternating current type plasma display, those produced by both a thin-film process using a vacuum system and a thick-film process using screen printing have begun to be put to practical use.
When the use of the plasma display in a color television with a large screen size is contemplated, the plasma display should have a memory function from the viewpoint of increasing the brightness. In this respect, the alternating current type plasma display inherently has a memory function by virtue of charges accumulated in the protective layer provided on the dielectric layer and, hence, is considered to be able to cope with a demand for an increase in screen size. Magnesium oxide having high secondary electron emission efficiency and excellent sputtering property has been used as a material for the protective layer. At the present time, a service life of 15000 hr has been achieved for a full-color, alternating current type plasma display, and a panel having a diagonal distance of 21 in. has been put on the market.
Methods for forming the protective layer include thin-film processes, such as EB deposition, sputtering, and CVD (Japanese Patent Publication Nos. 42579/1985 and 59221/1988), and thick-film processes, such as one that comprises spray-coating basic magnesium carbonate as a starting material for magnesium oxide on a substrate to form a thick coating and firing the coating to convert the basic magnesium carbonate to a metal oxide (Japanese Patent Publication No. 13983/1982) and one that comprises dispersing a fine powder of magnesium oxide in a liquid binder which, upon firing, can be converted to an oxide, thereby forming a magnesium oxide film (Japanese Patent Publication No. 283020/1994).
Among the above methods, those using a vacuum process, such as EB deposition, sputtering, and CVD, are disadvantageous in that it is difficult to accommodate a large panel substrate, like a plasma display, in a vacuum chamber, posing problems of cost of equipment and productivity when an increased screen size is assumed.
Coating is a simple method and, hence, has been extensively and intensively studied in the art. At the present time, however, no satisfactory performance could have been attained yet. The reason for this unsatisfactory performance is as follows. In the coating, a magnesium oxide printing paste containing magnesium oxide particles is used for printing a magnesium oxide protective layer for alternative current type PDP. In order that the protective layer has sputtering resistance, magnesium oxide should be in the form of homogeneous particles having a diameter of 30 to 300 nm and, at the same time, the particles should be homogeneously dispersed in a binder. However, fine particles of magnesium oxide are likely to agglomerate and become very difficult to be homogeneously dispersed in the binder. For this reason, in the conventional commercially available printing paste, the diameter of incorporated magnesium oxide particles per se is large and the viscosity of the paste per se is high, making it difficult to reduce the thickness of the protective layer. Consequently, no highly even protective film can be formed. Further, the conventional printing paste is disadvantageous in that the conventional heat treatment process (600.degree. C. or below) cannot offer satisfactory film strength, adhesion and other properties and heat treatment causes cracking of the film. The coatability of the paste is also unsatisfactory. Furthermore, the magnesium oxide particles incorporated are large, and the viscosity of the paste per se is high, making it difficult to reduce the layer thickness. This in turn poses a problem that a demand for minimized firing voltage and drive voltage cannot be met.
In order to solve the above problems involved in the coating method, the present inventors have previously proposed a method, using the so-called sol-gel process, that comprises the steps of: coating a magnesium hydroxide sol, prepared by hydrolyzing a magnesium oxide compound, onto a substrate; drying and firing the coating to form a magnesium oxide-containing thin film (Japanese Patent Laid-Open No. 111177/1996).
This method, however, is disadvantageous in that the adhesion of the magnesium hydroxide sol to the substrate is so poor that a part of the resultant magnesium oxide layer is easily separated from the substrate and the film thickness is uneven. Therefore, the above method is substantially ineffective in improving the firing voltage and the drive voltage (power consumption). Further, heat treatment of the magnesium hydroxide sol at a conventional temperature (600.degree. C. or below) does not accelerate the crystallization of the resultant magnesium oxide. Therefore, the film thus obtained has a low crystallinity and, hence, unsatisfactory film strength, causing a problem that properties as a protective film cannot be satisfactorily exhibited.
On the other hand, the use of an organometallic compound, such as a magnesium alkoxide, as a starting compound for the magnesium hydroxide sol can offer magnesium oxide having relatively good adhesion to the substrate and uniformity in the film. Magnesium alkoxides and the like involve a problem of cost, because they are very expensive, and, in addition, a problem associated with handling because the reactivity of the magnesium alkoxides is so high that the control of the reaction is difficult and the life of the resultant magnesium hydroxide is short.
Under the above circumstances, the present invention has been made, and an object of the present invention is to solve the above problems of the prior art and to provide a sol solution that can form a film on a panel having a large area without the need to use expensive equipment such as required in the vacuum process, the formed film having excellent film strength, adhesion, transparent, protective properties and other properties and being usable as a protective layer that can lower the firing voltage and the drive voltage (power consumption), and to provide a method for film formation using said sol solution.